Everything eventually goes to shit for the gang on Seinfeld, most of all relationships. Jerry, Kramer, Elaine, and especially George are borderline incapable of holding down long-term commitments (and that goes for jobs too), mostly due to their own narcissism, but sometimes, outside circumstances speed up the breakups.
The most recurring reason: food. Seinfeld and his crew just may have been the original foodies, and as they chased the newest NYC yogurt or soup hot spot, food was often a weapon used by or against them for revenge. Somehow, it all comes back to the kitchen.